The XF108 Rapier. A companion for my XB70

quorneng

Master member
Quite some time ago, actually 1974, I built a Depron XB70 Valkyrie.
Newcmplt1.JPG

Using a 70mm EDF that exhausting through 6 scale jet pipes.
Newcmplt3.JPG

Very light so an easy hand launch and belly land. To my surprise it actually flew very well and it still flies 10 year later.
I recently came across the XF108 mach 3 interceptor fighter also designed by North American.
Big3View.jpg
Obviously it shared some of the same characteristics so I wondered if it would fly as well. It certainly "looks the part".
It was actually never built although North American spent a lot of time and money on development as the need to intercept bombers disappeared when it became obvious that ICBMs would be the nuclear threat.
Unlike the XB70 where two physical examples were built the detail design of the XB108 was never finally fixed so there are significant differences in the available 3 view drawings. This means that what ever I build will be wrong!
North American did build a full scale "mock up" but as it was top secret at the time there are only a few pictures of it.
BigMockUp.jpeg
But it does give a 3 dimensional picture of what it would look like.
An important feature is the huge inlets. With the exhaust nozzle petals closed to mach 3 configuration the area of the inlet is quite adequate to feed an EDF. An unusual combination for a turbojet powered plane. I note that there are drawings that show much smaller inlets.
My idea is to build it like the XB70 as a light weight hand launch/belly lander out of 3mm XPS foam sheet using a pair of 40mm EDFs although those big under wing fins could be an issue.😟
We shall see.
 

quorneng

Master member
When building a light weight EDF from sheet foam my preferred method is to design and build the duct first and if possible go even further an built the plane around a complete duct that includes the EDF. I also have no qualms about placing the EDF well to the rear to ensure the battery is well forward and clear of the duct work hopefully somewhere in the nose.
The XF108 poses a few problems. The inlet is obviously a rectangle and the inlet to the EDF a circle and to add to the problem they are not inline. With a rear EDF the inlet is long so as a duct it has to be efficient. Any drop in inlet pressure or velocity has a serious impact on an EDF which is after all a very low pressure air accelerator rather than a pump.
The first task is to draw the duct outline to scale on a plan and elevation drawing to get an idea of its required shape and positioning.
PlanBig.jpg
ElevationBig.jpg
This generates both the full length and offset of the duct when scaled to a 40mm EDF.
I have a 3D printer and have found that CAD can produce really smooth shape variations that are mathematically exact so maintaining a specific cross section area. In this case the intent is to maintain 1.2 times the FSA of the 40mm EDF along the entire inlet duct.
The first CAD picture of one of the XF108 ducts.
RhDuctCAD.jpg

The gap is where the 40 mm EDF will go. I am sure there will be further iterations in the design when actual parts are printed.
The inlet has 3 components. The middle section handles the change from rectangle to a circle as well as the inlet to exhaust line offset This means it is "handed" to suite the left or right hand ducts. The front and rear parts are common to both ducts.
The XF108 has distinct exhaust nozzle "petals" to account for take off and mach 3 flight. As the exhaust nozzle will also be printed I wondered if the petal effect could be included particularly as there are 18 segments!
The first attempt using an 18 sided polygon hardly highlighted the line between the segments.
TestNozzle1.JPG

Note one benefit of printing is the inner profile is still a smooth circular duct.
I wondered instead of each segment being flat it would highlight the difference between then if each was slightly dished.
TestNozzle1+2.JPG

It certainly make the segments more visible.
The mathematics involved to create such a complex shape would be horrendous but fortunately the CAD package did all of it.
Far too many hours of work expanded and all I have to show for it is a "test" exhaust nozzle. :oops: To make matters worse I can't realistically print any part of the duct until the 40mm EDFs arrive from China!
Never mind. It is mid winter here in the UK so it will be several months before I will be able to actually test fly it.
 

quorneng

Master member
A bit of delay as I decided te XF108 needed to be 50% bigger but still retaining the 40 mm EDFs.
The 50% bigger inlets look ..er.. much bigger!
BigInlet.JPG

The bigger centre box of the XF108 with one EDF in place with its exhaust nozzle.
Ducts+1EDF.JPG

As the 40 mm EDF is retained it does mean the scale nozzle "petals" have to be taken down to a diameter smaller than would ever have been used on te full size.
The 12 blade 40mm fan viewed from the rear.
EDFnozzle.JPG

The nozzle exit is 90% of the FSA. The whole inlet duct is relatively generous 1.2 times the FSA. Definitely no cheat holes required!
Still a long way to go.